Due to the recent upsurge in data traffic, communication networks have increasingly large transfer capacities. In a communication network, especially in a transport network forming its core, one of the most important issues is to improve reliability.
A transport network accomplishes data communications using a traffic transfer path which is made up of nodes and links. Thus, a network will fail to transfer data when a failure occurs at any node or link. Reliability of a transport network is expressed by a period from the time an active transmit/receive node becomes unable to communicate to the time it is enabled again, i.e. a period for a path to recover from failure. In general transport networks, a path is required to recover from failure within 50 milliseconds.
Mechanisms for a transport network to recover from failure include two schemes: protection and restoration.
In a protection scheme, a backup path which does not cross the working path used for normal traffic transfer is preset, and in case of a failure on the working path, transmit/receive nodes are switched so that the path used for traffic transfer is switched to the backup path. For example, the Ethernet® Linear Protection scheme as defined in ITU-T G.8031 is generally known. This scheme allows automatic switching caused by failure as well as manual switching for maintenance, etc.
In a restoration scheme, a backup path is defined after a failure occurs on the working path, and thus not only transmit/receive nodes but also relay nodes are switched. In general, a restoration scheme includes a scheme where a path is redefined via NMS (Network Management System) after detection of a failure, and also includes a scheme where individual nodes autonomously redefine a path by using a path definition protocol. A restoration scheme allows to continue data communication as far as a fault-free path exists even if a failure occurs simultaneously both on the working and backup paths.
If a restoration scheme is used, a time required for a path to recover from failure is the same as that for additionally defining one new path. Thus, a path failure recovery time based on a restoration scheme is often longer than 50 milliseconds, which is the time required in a transport network as a path failure recovery time.
Accordingly, a protection scheme is preferable in order to quickly recover from a failure.
Some network users such as government agencies and banks demand particularly high reliability. Thus, there exists the need for a mechanism which allows fast and reliable recovery of user data even when a failure occurs both on the working and backup paths. Consequently, a transport network is required to provide two or more backup paths for one working path.
PTL 1 and PTL 2 each disclose a method for quickly switching between paths in a communication system that provides two or more backup paths for one working path.
PTL 1 discloses a technique for switching to a backup path in case of a failure on the working path by selecting two fault-free paths from among three or more paths and carrying out a conventional protection protocol between the two paths.
PTL 2 discloses a technique for defining four paths as a first working path, a first backup path, and a second working path, a second backup path, respectively, and carrying out a conventional protection protocol between the first working path and the first backup path and between the second working path and the second backup path.